Phenalenone (PN) derivatives are involved in plant defense strategies, producing molecular singlet oxygen in a photosensitization process. Many experimental and theoretical studies determined that PN can performe this process with a quantum yield close to 1. However, it has been observed that the efficiency of some of its derivatives is much lower. This is the case of 9-phenylphenalenone (9-PhPN). To elucidate the factors that determine the different photochemistry of PN and its derivate 9-PhPN, we developed a complete active space self-consistent field/multi-configurational second-order perturbation theory study where several deactivation paths through the lowest excited states were explored. We found that the characteristics of the low-lying excited states are similar for both PN and 9-PhPN in the areas near the geometry of excitation. Consequently, the first processes that take place immediately after absorption are possible in both systems, including the population of the triplet state responsible for oxygen sensitization. However, 9-PhPN can also undergo cyclization by a bond formation between the carbonyl oxygen and a carbon atom of the phenyl substituent. This process competes favorably with population of triplet states and is responsible for the decrease of the quantum yield of oxygen sensitization in 9-PhPN relative to PN.

中文翻译：

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光敏化与光环化：苯菲酮在植物防御策略中作为抗植物激素的竞争反应

苯酚酮（PN）衍生物参与植物防御策略，在光敏化过程中产生分子单线态氧。许多实验和理论研究确定PN可以以接近1的量子产率执行该过程。但是，已经观察到其某些衍生物的效率要低得多。9-苯基苯甲酮（9-PhPN）就是这种情况。为了阐明决定PN及其衍生物9-PhPN不同光化学的因素，我们开发了完整的有源空间自洽场/多构型二阶微扰理论研究，其中探索了通过最低激发态的几种失活路径。我们发现，在接近激发几何的区域中，PN和9-PhPN的低激发态的特征都相似。因此，在两个系统中都可能发生吸收后立即发生的第一个过程，包括负责氧敏化的三重态的总体。但是，9-PhPN也可以通过羰基氧和苯基取代基的碳原子之间的键形成而环化。该过程与三重态的种群竞争性良好，并且是相对于PN降低9-PhPN中氧敏化的量子产率的原因。